1. Field of the Invention
The present invention relates to battery packs for portable devices. More particularly, though not exclusively, the present invention relates to a method and apparatus for providing a flexible circuit board with combined mechanical and electrical points of connection to other components in an electromechanical system.
2. Problems in the Art
Prior art flexible printed circuit boards typically consist of copper foil laminated between layers of flexible, non-conductive film. Voids in either layer of the non-conductive film expose the copper foil and serve as attachment points, via soldering, for the various electronic components which will be connected to the circuit. By selectively removing portions of the copper film during manufacturing, separate current carrying paths, or traces, are created in the foil. The traces connect various electronic components mounted on the circuit board.
Prior art circuit boards are typically designed for a specific application, and may be usable in other similar applications if the arrangement and/or content of electronic components mounted on the circuit board are varied to accommodate the other application. This variation causes the creation of circuit board designs which are distinct from each other and are not interchangeable. Each distinct design functions in a way unique to the configuration of the circuit traces and electronic components. As a result, a unique circuit board design exists for each unique functional application. This creates the need to design, manufacture, and inventory each of the unique circuit board designs which results in higher costs (mainly due to lower purchasing volumes and increased inventory carrying costs) for manufacturers utilizing these circuit boards in their products.
The basic components of a typical rechargeable battery pack include 2-12 rechargeable series-connected cells, other related electronic components, and a plastic case enclosing the cells and components. The battery pack case is usually fitted with metal contacts (device contacts) which serve as points of electrical connection between the cells inside the case an electrical power consuming device on the outside of the case. In addition, the metal contacts (charge contacts) serve as points of electrical connection between the cells and an electrical power supplying device (charger) on the outside of the case for the purpose of recharging the battery cells. Other common electronic components in a battery pack include a thermister for monitoring the temperature inside the battery pack and a diode for preventing the accidental discharge of the cells through the charge contacts.
FIG. 1 is an electrical schematic diagram of a typical prior art battery pack. As shown, a number of cells 10 are electrically connected to a positive current trace 12 and to a negative current trace 14 via cell contacts 13 and 15, respectively. The traces 12 and 14 are electrically connected to a pair of device contacts 16, 18 and a pair of charge contacts 20, 22.
The capacity and type of the cells are used by the charging device to determine the ideal rate of charging current delivered to the cells. Many charging devices are designed to charge battery packs with various types of cells. A common method used to enable the charger to determine the charging requirements of the cells in a given battery pack involves the use of a sense resistor having a specific resistance.
FIG. 2 is an electrical schematic diagram of a battery pack utilizing a sense resistor R.sub.s. The charging device (not shown) connects to the battery pack via charge contacts 20 and 22. The charging device also connects to a charger sense resistor contact 24. The charging device is then capable of detecting the resistance value of the sense resistor R.sub.s in the battery pack. The charging device is programmed to provide an appropriate charging current for the pack depending on the value of the sense resistor R.sub.s. For example, a battery pack having a low capacity may be built with a sense resistor R.sub.s of a specific value (e.g., 20k .OMEGA.), while a battery pack having a high capacity may be built with a sense resistor R.sub.s of a specific, but different, value (e.g., 10k .OMEGA.).
FIG. 3 shows one embodiment of a flexible circuit board 26 designed in accordance with FIG. 2. The circuit board 26 is comprised of copper foil 27 (FIG. 5) disposed between sheets of flexible non-conductive film 28 (FIG. 5). The copper foil further comprises positive and negative current traces 12 and 14 which connect to cells (not shown) via cell contacts 13 and 15, respectively. The traces 12 and 14 connect to a device (not shown) via the device contacts 16 and 18, respectively. The traces 12 and 14 also connect to a charging device (not shown) via the charge contacts 20 and 22, respectively. The charging device also connects to the sense resistor R.sub.s via the sense resistor contact 24. The contacts 16, 18, 20, 22, and 24, are comprised of circular portions of the copper foil with no flexible non-conductive film disposed thereon, therefore exposing the foil on one side of the circuit board. A hole is formed in the center of each contact.
A common method of designing external contacts is to use rivets to secure the circuit board to the battery case. FIGS. 4-7 illustrate how rivets 32 are used to secure the flexible circuit board 26 to the battery pack case 34, while, at the same time, providing an electrical contact point. The head of the rivets 32 form the device/charger interface surface on the outside of the battery pack case 34 by passing through the plastic case 34 and through the flexible circuit board 26.
FIG. 4 is an exploded view of the circuit board 26, the plastic battery pack case 34, a rivet 32 and a washer 36. As shown, the case 34 include a number of holes 38, 40, 42, 44, and 46 which correspond to the holes formed in the contacts 16, 18, 20, 22, and 24, respectively. When the circuit board 26 is inserted into the case 34, rivets 32 are inserted through the holes and contacts.
FIGS. 5-7 are enlarged sectional views illustrating the insertion of the rivet 32 through a hole forming a contact. All of the contacts are formed in an identical manner. As shown, a rivet 32 is inserted through the hole formed in the case 34, the circuit board 26, and the washer 36. FIG. 6 shows a staking tool 48 which is used to secure the rivet 32 in place. By subjecting the shank of the rivet 32 to a staking process, the shank of the rivet 32 is deformed (FIG. 7) so that it overlaps onto the non-insulated portion of the copper foil 27 on the flexible circuit board 26. The copper foil 27 is therefore electrically connected to the rivet 32. To further improve the integrity of the electrical contact between the trace and the overlapping rivet 32, the washer 36 (flat or toothed) is located between the rivet and the copper foil to increase the surface area available to conduct current through the junction.